1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using a photoreceptor and coherent light such as laser light for writing a latent image on the photoreceptor. The present invention also relates a photoreceptor for the image forming apparatus.
2. Discussion of the Background
Electrophotographic image forming methods using coherent light such as laser light for writing an electrostatic latent image on a photoreceptor are widely used for digital image forming apparatus such as copiers, printers and facsimile machines.
The image forming methods using coherent light for writing a latent image on a photoreceptor have a drawback such that the resultant images have a stripe image having uneven density because the coherent light interferes in the photosensitive layer of the photoreceptor. It is known that such undesired stripe images are produced when the following relationship is satisfied:
2nd=mxcex
wherein n represents the refractive index of the photosensitive layer when measured by the light used for image writing; d represents the thickness of the photosensitive layer; xcex represents a wavelength of the light used for image writing; and m is an integer. This is because the light is strengthened by the interference when such a relationship is satisfied. For example, when xcex is 780 nm and n is 2.0, a pair of a dark line image and a light line image are generated when the thickness of the photosensitive layer varies by 0.195 xcexcm. It is hard to control the thickness variation in the entire photosensitive layer within 0.195 xcexcm. Therefore, in attempting to solve the undesired stripe image problem, the following methods have been proposed:
(1) Japanese Laid-Open Patent Publication No. 57-165845 discloses a photoreceptor which includes an aluminum substrate and amorphous silicon formed thereon and serving as a charge generation layer, wherein a light absorbing layer is formed on the aluminum substrate to avoid specular reflection on the surface of the aluminum substrate. This method is effective for a photoreceptor having a layer structure in which a charge transport layer and a charge generation layer (an amorphous silicon layer) are formed on an aluminum substrate in this order. However, the method is hardly effective for a photoreceptor, which is typically used for electrophotographic image forming apparatus and which has a layer structure in which a charge generation layer (an amorphous silicon layer) and a charge transport layer are formed on an aluminum substrate in this order.
(2) Japanese Laid-Open Patent Publication No. 7-295269 discloses a photoreceptor which has a layer structure in which an undercoat layer, a charge generation layer and a charge transport layer are formed on an aluminum substrate in this order, wherein a light absorbing layer is formed on the aluminum substrate. However, this method cannot perfectly prevent the occurrence of the undesired stripe image.
(3) Japanese Patent Publication No. 7-27262 discloses an image forming apparatus which includes a photoreceptor including a cylindrical substrate having a cross section in which a main peak is overlapped with a sub-peak when the substrate is cut along a plane including the central axis of the cylindrical substrate. The apparatus has a light irradiator which irradiates the photoreceptor with coherent light beam whose diameter is less than the one cycle of the main peak, to form a latent image. This image forming apparatus can prevent the occurrence of the undesired stripe image when a limited photoreceptor is used. However, there are many photoreceptors which have such a substrate but which produce undesired stripe images even when used for the image forming apparatus.
(4) Japanese Laid-Open Patent Publication No. 10-301311 discloses a photoreceptor whose surface has a Arithmetical Mean Deviation of the Profile (Ra) (defined in JIS B0601), wherein the Ra is not less than the wavelength of the light used for image writing. When the formed image has a low resolution or the light used for image writing has a relatively large diameter, the occurrence of the undesired stripe image can be almost prevented by this method. However, when the diameter of the light becomes small to record images having high resolution, the undesired stripe image is often generated. It can be said that the method of controlling the surface roughness of a substrate in view of Ra is effective only for substrates whose surface is represented by a wave which has a similar amplitude. However, such substrates are rare, and almost all the substrates have a surface represented by a wave in which plural waves having different amplitudes and different wavelengths are overlapped with each other. Ra is not proper for representing the roughness of such substrates because the waves other than the waves having a relatively large amplitude are cancelled when Ra of a profile is measured.
The surface roughness of a substrate can be represented by another parameter such as Ry (maximum height) and Rz (ten-point mean roughness) defined JIS B0601. However, even when the surface roughness of a substrate is controlled by controlling such parameters, the occurrence of the undesired stripe images cannot be perfectly avoided. In particular, when the diameter of the light used for image writing becomes small to obtain high resolution images, the undesired stripe images are often produced.
Because of these reasons, a need exists for an image forming apparatus using a photoreceptor, which can produce good images without an undesired stripe image caused by specular reflection in the photoreceptor.
Accordingly, an object of the present invention is to provide an image forming method and apparatus using a photoreceptor, which can produce good images without an undesired stripe image caused by specular reflection in the photoreceptor.
Another object of the present invention is to provide a photoreceptor which can produce good images without an undesired stripe image caused by specular reflection therein.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image forming apparatus including a photoreceptor having a photosensitive layer overlying an electroconductive substrate and a light irradiator configured to irradiate the photoreceptor with coherent light having a wavelength xcex represented in units of micrometers and a diameter xcfx86 represented in units of micrometers, wherein a maximum height in a part of the profile of the lower surface (i.e., the surface closer to the substrate) of the photosensitive layer in a sampling range of xcfx86 is not less than xcex/(2n), where n is a refractive index of the photosensitive layer at the wavelength xcex.
The photoreceptor may include an undercoat layer between the photosensitive layer and the substrate. In this case, the profile of the upper surface of the undercoat layer has the property mentioned above.
Alternatively, a maximum height in any part of the profile of the surface of the substrate is not less than {xcex/(2n)}xc3x971.03 in a sampling range of xcfx86. In this case, an undercoat layer maybe formed between the photosensitive layer and the substrate, which preferably has a thickness not greater than 15 xcexcm.
The diameter of the coherent light is preferably not greater than xcex xcexcm.
The refractive index of the photosensitive layer is preferably from 1.2 to 2.0 when measured by the light having a wavelength of xcex xcexcm.
The image forming apparatus may further include a charger which charges the photoreceptor before writing the latent image, an image developer having plural developing station each including a different color developer. The color image forming apparatus may have plural photoreceptors for forming a different color image thereon. In addition, the color image forming apparatus preferably has an intermediate transfer belt on which different color images are transferred from the photoreceptor or photoreceptors to form a color image. The color image is then transferred onto a receiving material.
In another aspect of the present invention, a photoreceptor is provided which is used for an image forming apparatus and which includes a photosensitive layer located on an electroconductive substrate, wherein a maximum height of a part of the profile of the lower surface of the photosensitive layer is not less than xcex/(2n) in a sampling range of xcfx86, wherein xcfx86 and xcex represent the diameter (xcexcm) and wavelength (xcexcm) of light used for image writing and n is the refractive index of the photosensitive layer at the wavelength xcex.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.